Few-cycle Femtosecond Laser Research Articles

Spatiotemporal dispersion compensation for a 200-THz noncollinear optical parametric amplifier.

A noncollinear optical parametric amplifier (NOPA) can produce few-cycle femtosecond laser pulses that are ideally suited for time-resolved optical spectroscopy measurements. However, the nonlinear-optical process giving rise to ultrabroadband pulses is susceptible to spatiotemporal dispersion problems. Here, we detail refinements, including chirped-pulse amplification (CPA) and pulse-front matching (PFM), that minimize spatiotemporal dispersion and thereby improve the properties of ultrabroadband pulses produced by a NOPA. The description includes a rationale behind the choices of optical and optomechanical components, as well as assessment protocols. We demonstrate these techniques using a 1kHz, second-harmonic Ti:sapphire pump configuration, which produces ∼5-fs duration pulses that span from about 500 to 800nm with a bandwidth of about 200THz. To demonstrate the utility of the CPA-PFM-NOPA, we measure vibrational quantum beats in the transient-absorption spectrum of methylene blue, a dye molecule that serves as a reference standard.

Full Temporal Characterization of Ultrabroadband Few-Cycle Laser Pulses Using Atomically Thin WS2

Nowadays, the accurate and full temporal characterization of ultrabroadband few-cycle laser pulses with pulse durations below 7 fs is of great importance in fields of science that investigate ultrafast dynamic processes. There are several indirect methods that use nonlinear optical signals to retrieve the complex electric field of femtosecond lasers. However, the precise characterization of few-cycle femtosecond laser sources with an ultrabroadband spectrum presents additional difficulties, such as reabsorption of nonlinear signals, partial phase matching, and spatiotemporal mismatches. In this work, we combine the dispersion scan (d-scan) method with atomically thin WS2 flakes to overcome these difficulties and fully characterize ultrabroadband laser pulses with a pulse duration of 6.9 fs and a spectrum that ranges from 650 to 1050 nm. Two-dimensional WS2 acts as a remarkably efficient nonlinear medium that offers a broad transparency range and allows for achieving relaxed phase-matching conditions due to its atomic thickness. Using mono- and trilayers of WS2, we acquire d-scan traces by measuring the second-harmonic generation (SHG) signal, originated via laser–WS2 interaction, as a function of optical dispersion (i.e., glass thickness) and wavelength. Our retrieval algorithm extracts a pulse duration at full-width half-maximum of 6.9 fs and the same spectral phase function irrespective of the number of layers. We benchmark and validate our results obtained using WS2 by comparing them with those obtained using a 10-μm-thick BBO crystal. Our findings show that atomically thin media can be an interesting alternative to micrometer-thick bulk crystals to characterize ultrabroadband femtosecond laser pulses using SHG-d-scan with an error below 100 as (attoseconds).

Water jet space charge spectroscopy: route to direct measurement of electron dynamics for organic systems in their natural environment

The toolbox for time-resolved direct measurements of electron dynamics covers a variety of methods. Since the experimental effort is increasing rapidly with achievable time resolution, there is an urge for simple and robust measurement techniques. Within this paper prove-of-concept experiments and numerical simulations are utilized to investigate the applicability of a new setup for the generation of ultrashort electron pulses in the energy range of 300 eV up to 1.6 keV. The experimental approach combines an in-vacuum liquid microjet and a few-cycle femtosecond laser system, while the threshold for electron impact ionization serves as a gate for the effective electron pulse duration. The experiments prove that electrons in the keV regime are accessible and that the electron spectrum can be easily tuned by laser intensity and focal position alignment with respect to the water jet. Numerical simulations show that a sub-picosecond temporal resolution is achievable.

Simulation of the photoionization of nonmetal crystals by few-cycle femtosecond laser pulses

Modeling of laser-induced photoionization is a key component in simulations of ultrafast laser ablation of transparent solids. Available analytical and phenomenological models consider the ionization by promoting valence band electrons to a conduction band via simultaneous absorption of several laser photons of the same energy defined at central wavelength of laser-pulse spectrum. That assumption corresponds to nearly zero spectrum bandwidth and is not true for ultrashort few-cycle pulses. Treatment of the few-cycle-pulse photoionization by numerical methods, e.g., time-dependent density-functional theory, meets substantial difficulties when modeling of ionization-rate scaling with multiple laser-pulse and material parameters. To address those gaps, we report an analytical approach to evaluation of the photoionization rate by a few-cycle laser pulse with nonnegligible spectral bandwidth. We assume that the pulse spectrum supports the few-cycle pulse duration, but is narrow enough to prevent quantum interference between multiphoton transitions of different orders. We outline the calculation procedure, report examples of simulations with the proposed model, and discuss some novel physics of the photoionization. Our model includes dependence of the photoionization rate on carrier-envelope phase and delivers significantly higher rates compared with the Keldysh model. We interpret those results in terms of laser-driven variations of effective band gap, opening of extra channels of the multiphoton absorption, and involvement of a continuous range of electron states determined by pulse spectrum width. Based on the reported model, we discuss new options to control threshold of ultrafast laser ablation via carrier-envelope and pulse-shape scaling of the photoionization rate.

GV m−1 on-chip particle accelerator driven by few-cycle femtosecond laser pulse

Particle accelerator on chip with high acceleration gradient has been an unremitting goal of researchers. Dielectric laser accelerator (DLA) is a possible candidate to achieve this goal. However, due to the limitation of dielectric breakdown, it is difficult for the available DLAs to reach an acceleration gradient as high as 1 GV m−1 since a long-duration multi-cycle laser pulse with high fluence have to be used. Here we propose to use a few-cycle laser pulse to drive a DLA based on the inverse Cherekov radiation effect. It significantly reduces the required pulse duration and the laser fluence, remarkably increasing the achievable acceleration gradient. Moreover, by using a cascade acceleration scheme, we realize a high energy-gain acceleration for low-energy electrons in a microscale device by simulation, which paves the way for the development of a fully on-chip particle accelerator.

Generation of attosecond electron bunches and x-ray pulses from few-cycle femtosecond laser pulses

Laser-plasma electron accelerators can be used to produce high-intensity x-rays, as electrons accelerated in wakefields emit radiation due to betatron oscillations. Such x-ray sources inherit the features of the electron beam; sub-femtosecond electron bunches produce betatron sources of the same duration, which in turn allow probing matter on ultrashort time scales. In this paper we show, via Particle-in-Cell simulations, that attosecond electron bunches can be obtained using low-energy, ultra-short laser beams both in the self-injection and the controlled injection regimes at low plasma densities. However, only in the controlled regime does the electron injection lead to a stable, isolated attosecond electron bunch. Such ultrashort electron bunches are shown to emit attosecond x-ray bursts with high brilliance.

Focus on photonics, spectrometry, and spectroscopy

Focus on photonics, spectrometry, and spectroscopy

Investigation of ultra-broadband terahertz generation from sub-wavelength lithium niobate waveguides excited by few-cycle femtosecond laser pulses.

The generation of coherent, ultra-broadband terahertz (THz) radiation pulses spanning more than a few octaves is vital to understanding the ultrafast response of elementary excitations, molecules, nanostructures, materials, and explore device functionality across a wide spectrum. In this work, we use 2D finite-difference time-domain simulations to show that ultra-broadband (0.18-106 THz) Cherenkov radiation can be produced from SiO2:MgO-LiNbO3:SiO2 waveguides having core dimensions that are sub-wavelength with respect to the optical pump pulse being guided. These sub-wavelength core dimensions allow the ultra-broad Cherenkov radiation to be emitted at an angle between 47.2° and 47.5° (dictated by the Si cladding layer dispersion), making these waveguide structures superior to the THz generation arrangements in bulk MgO-LiNbO3 crystals. When excited by a 7 fs, 780nm laser pulse having an energy of 2 nJ, a 300 µm-long waveguide with transverse core dimensions of 500 nm × 2 mm can generate a sub-ps, kV/cm electric field pulse. Unlike THz pulse generation in bulk MgO-LiNbO3 crystals, having sub-wavelength core dimensions reduce the absorption from the MgO-LiNbO3 reststrahlen bands. These sub-wavelength SiO2:MgO-LiNbO3:SiO2 waveguides are ideal for on-chip applications that require ultra-broadband, compact THz sources.

Electron dynamics and optical properties modulation of monolayer MoS2 by femtosecond laser pulse: a simulation using time-dependent density functional theory

In this study, we adopted time-dependent density functional theory to investigate the optical properties of monolayer MoS2 and the effect of intense few-cycle femtosecond laser pulses on these properties. The electron dynamics of monolayer MoS2 under few-cycle and multi-cycle laser irradiation were described. The polarization direction of the laser had a marked effect on the energy absorption and electronic excitation of monolayer MoS2 because of anisotropy. Change in the polarization direction of few-cycle pulse changed the absorbed energy by a factor over 4000. Few-cycle pulse showed a higher sensitivity to the electronic property of material than multi-cycle pulse. The modulation of the dielectric properties of the material was observed on the femtosecond time scale. The negative divergence appeared in the real part of the function at low frequencies and photoinduced blue shift occurred due to Burstein-Moss effect. The irradiation of femtosecond laser caused the dielectric response within the infrared region and introduced anisotropy to the in-plane optical properties. Laser-based engineering of optical properties through controlling transient electron dynamics expands the functionality of MoS2 and has potential applications in direction-dependent optoelectronic devices.

Publisher's Note: Comment on “Direct photodetachment of F− by mid-infrared few-cycle femtosecond laser pulses” [Phys. Rev. A 94 , 057401 (2016)

Received 19 December 2016DOI:https://doi.org/10.1103/PhysRevA.94.069903©2016 American Physical Society

Nonsequential double ionization of Ne subject to few-cycle femtosecond laser pulses

We investigate experimentally and theoretically the evolution of Ne2+ ion momentum distributions from Nonsequential Double Ionization (NSDI) in few-cycle laser pulses with respect to the Carrier Envelope Phase (CEP). The distributions depend strongly on the CEP and exhibit an asymmetric double-hump structure at some CEPs. A semiclassical calculation can reproduce the experimental observations qualitatively, where the rescattering scenario is identified as the main mechanism of double ionization. Analysis based on this semiclassical model reveals that double ionization happens a little later than the collision. Depending on the time difference between the ionization moment and the collision moment, we can distinguish two ionization pathways involved, i.e., Recollision Impact Ionization (RII) (with time difference below 0.1 optical cycle) and Recollision-Excitation and Subsequent Ionization (RESI) (with time difference above 0.1 optical cycle) pathways. The asymmetric pattern results from both RII and RESI: RII leads to the asymmetric double-hump structure, while RESI contributes to the low momentum part and makes the dip of the double-hump structure shallower. Furthermore, Ne2+ ion momentum distributions from RII depend strongly on the CEP and the distributions from RESI seem to be not very sensitive to the CEP. To shed more light on physical mechanism of the asymmetric momentum distribution, the momentum distributions from two ionized electrons are presented at the collision moment, the ionization moment and the moment right after laser field ends for each pathway, respectively. The comparison unravels that the asymmetry comes from the asymmetric acceleration from the laser electric field after the double ionization moments. To demonstrate the significant role of long range Coulomb potential in NSDI process, we replace the Coulomb potential with a short-range model potential in the calculation and the results show appreciable distinctions from the case of Coulomb potential. It is clarified that the Coulomb potential raises the yields of RESI process, which, in turn, makes the dip shallower.

Comment on “Direct photodetachment of F− by mid-infrared few-cycle femtosecond laser pulses”

Multiphoton detachment of F$^-$ by strong few-cycle laser pulses was studied by Shearer and Monteith using a Keldysh-type approach [Phys. Rev. A 88, 033415 (2013)]. We believe that this work contained errors in the calculation of the detachment amplitude and photoelectron spectra. We describe the necessary corrections to the theory and show that the results, in particular, the interference features of the photoelectron spectra, appear noticeably different.

Carrier-envelope phase-dependent electronic conductivity in an air filament driven by few-cycle laser pulses

The modulation of the electron conductivity in an air filament, which is produced by carrier-envelope phase (CEP) stabilized 7-fs laser pulses, is realized experimentally. Numerical results based on a coupled 3D+1 generalized nonlinear Schr\odinger equation including the real electric-field dependent ionization model are in good agreement with those from the experiment. It is demonstrated that the CEP effect on the electron density originates from the CEP-induced modification of the electric field of the laser pulse, and this modification is amplified during nonlinear propagation. The results provide important information to help understand the physical mechanism of the filaments driven by few-cycle femtosecond laser pulses.

Photodetachment of Si− by mid-infrared few-cycle femtosecond laser pulses

The Kelydsh model originally developed for infinitely long periodic laser pulses is extended to a spin polarized model to consider direct photodetachment of negative ions with half-filled np3 valence shells by few-cycle laser pulses without taking into account rescattering effects. The theory is applied to study above threshold detachment of Si− negative ions by intense short infrared pulses but is expected to be inherent of any half-filled p subshell negative ion. Photoelectron momentum maps of the direct electrons of Si− are presented. These are found to exhibit a distinctive threshold structure of concentric elliptical rings where the separation of these rings is dependent on the quantized frequency components of the few-cycle laser pulse. Photoelectron angular distributions (PADs) predicted by short duration laser pulses are calculated and threshold effects associated with channel closings are observed as the laser-field frequency and intensity pass through critically induced ponderomotive potential channel closures. Enhanced structures of the PADs in the vicinity near threshold show that the detachment process does not follow the Wigner law. This is a because the photon energy is not conserved in a few-cycle pulse in contrast to a long periodic pulse. In the energy spectra interference effects are observable as multiphoton peaked structures whose positions are dependent on the ponderomotive threshold shifts determined by the nature of the time-dependent vector potential. Additionally we show that carrier envelope phase effects manipulate all emission spectra considered.

High-power table-top white-light few-cycle laser generator

We have achieved experimentally a generation of white-light few-cycle femtosecond laser pulses, using a neon-filled hollow-core fibre. The pulses observed...

Direct photodetachment of F−by mid-infrared few-cycle femtosecond laser pulses

The recent adiabatic saddle-point approach of Shearer et al. [Phys. Rev. A 84, 033409 (2011)] is extended to multiphoton detachment of negative ions with outer $p$-state electrons. This theory is applied to investigate the strong-field photodetachment dynamics of F${}^{\ensuremath{-}}$ ions exposed to few-cycle femtosecond laser pulses, without taking into account the rescattering mechanism. Numerical calculations are considered for mid-infrared laser wavelengths of 1300 and 1800 nm at laser intensities of 7.7 $\ifmmode\times\else\texttimes\fi{}$ 10${}^{12}$, 1.1 $\ifmmode\times\else\texttimes\fi{}$ 10${}^{13}$, and 1.3 $\ifmmode\times\else\texttimes\fi{}$ 10${}^{13}$ W/cm${}^{2}$. Two-dimensional momenta saddle-point spectra exhibit a distinct distribution in the shape of a ``smile'' in the complex-time plane. Electron momentum distribution maps of direct electrons are investigated. These produce a distinct pattern of above-threshold detachment (ATD) concentric rings due to constructive and destructive quantum interference of electrons detached from their parent ions. Probability detachment distributions presented, capturing the influence of saturation effects that are found to become more significant with increasing laser intensity at a fixed wavelength. ATD photoangular distributions as functions of laser intensity and wavelength near channel closings are also investigated and found to be sensitive to initial-state symmetry. Nonmonotonic structures observed in the ejected photoelectron energy spectra are attributed to interference effects from coherent electronic wave packets. Additionally the profiles of all the photoelectron emission spectra show strong dependence on the carrier-envelope phase, indicating that it is a reliable parameter for characterizing the wave form of the pulse.

THz emission control by tuning density profiles of neutral gas targets during intense laser-gas interaction

Ionization currents generated from two-color or few-cycle intense femtosecond laser pulses interaction with neutral gas targets can emit strong THz waves. Here it is found that the initial non-uniformity of the gas density can significantly affect the ionization currents and subsequent THz emission both in amplitude and in spectrum. Density profile effects on the forward and backward emissions have been studied in details by particle-in-cell simulations, in which the field ionization module is included. Increasing the gas density gradient length, the emitted forward THz spectrum shifts from high to low frequency, and the spectrum width reduces, which offers a way to obtain a tunable THz emission source by laser-gas interaction.

Generation of isolated attosecond pulses using a plasmonic funnel-waveguide

We theoretically investigated the possibility of generating attosecond pulses by means of plasmonic field enhancement induced in a nano-structured metallic funnel-waveguide. This study was motivated by our recent experimental demonstration of ultrashort extreme-ultraviolet (EUV) pulses using the same type of three-dimensional waveguides. Here, with emphasis on generation of isolated attosecond pulses, the finite-domain time-difference method was used to analyze the funnel-waveguide with respect to the geometry-dependent plasmonic features such as the field enhancement factor, enhanced plasmonic field profile and hot-spot location. Then an extended semi-classical model of high-order harmonic generation was adopted to predict the EUV spectra generated from the funnel-waveguide in consideration of the spatial inhomogeneity of the plasmonic field within the hot-spot volume. Our simulation finally proved that isolated attosecond pulses can be produced at fast repetition rates directly from a few-cycle femtosecond laser or by synthesizing a two-color laser consisting of two multi-cycle pulses of cross-polarized configuration.

Frequency tuning enhancement of few-cycle femtosecond laser pulses by double-pump-induced N_2 index modulation

Considering field-free molecular alignment enhancement by multiple pulses in the hollow fiber, we theoretically analyze how long-distance propagation influences the final alignment degree. Based on this, we propose a way to enhance frequency tuning of few-cycle femtosecond laser pulse by multi-pump-induced N2 index modulation. It is found that two pumps are optimized with parameters of 100 fs and 3×1013 W cm−2. The tuning range of the initial 5 fs 800 nm probe pulse by double aligning pumps is about twice that by a single pump.

Nonponderomotive electron acceleration in ultrashort surface-plasmon fields

We investigate the nonponderomotive nature of ultrafast plasmonic electron acceleration in strongly decaying electromagnetic fields generated by few-cycle and single-cycle femtosecond laser pulses. We clearly identify the conditions contributing to nonponderomotive acceleration and establish fundamental scaling laws and carrier-envelope phase effects. These all-optically accelerated compact, femtosecond electron sources can be utilized in contemporary ultrafast methods.